JP2013064858A - Optical system and optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical system and an optical element in which a refraction power in a diffraction plane of a diffraction optical part is alleviated so as to suppress a flare due to unnecessary diffraction light by making other optical elements in a diffraction optical element share an effect of reducing chromatic aberration contained in the diffraction optical element itself and an aspheric lens effect, as a further improvement of a conventional technology.SOLUTION: The optical element is formed by joining the diffraction optical section formed by laminating a plurality of diffraction gratings, and a refraction optical section comprising a solid material having an abnormal partial dispersion characteristic. The refraction optical section includes an optical plane which is not joined to the diffraction optical section, exposed to the air, and has an aspherical shape.

Description

  The present invention relates to a diffractive optical element and an optical system using the diffractive optical element, and is suitable for a photographing optical system such as a silver salt photographic camera, a video camera, and a digital still camera.

  In general, in an optical system used for an imaging apparatus such as a digital camera or a video camera, as the overall length of the lens is shortened and the entire optical system is reduced in size and weight, various aberrations, in particular, on-axis and lateral chromatic aberration are deteriorated, and optical performance is improved. It tends to decrease. In particular, in a telephoto type optical system in which the total lens length is shortened, the chromatic aberration tends to deteriorate as the focal length increases.

  As a method for reducing the occurrence of such chromatic aberration, a method using an abnormal partial dispersion material as an optical material and a method using a diffractive optical element in an optical path are known (Patent Document 1). Regarding the former anomalous partial dispersion material, ordinary optical glass has the property that the Abbe number is plotted on a horizontal axis while the following partial dispersion ratio is plotted on a vertical axis on a certain straight line (normal). (Partial dispersion) However, it means that it does not ride on a straight line. Here, the partial dispersion ratio is the refractive index of the other two wavelengths with respect to the difference in refractive index (main dispersion) at two wavelengths (for example, the F-line and C-line of the Fraunhofer line) serving as the reference of the optical material. Ratio (partial dispersion).

  Further, with respect to the latter diffractive optical element, the chromatic aberration is reduced by the fact that the emission angle relationship of light of different wavelengths by the diffraction grating having the diffractive action is opposite to the emission angle relationship by the optical surface having the refracting action. This diffractive optical element is known to have an aspherical lens effect by changing the period of its periodic structure (Patent Document 2).

  Patent Document 1 discloses an optical element and an optical system in which the above-described refractive optical unit having abnormal partial dispersion and a diffractive optical unit formed by stacking a plurality of diffraction gratings are joined.

JP2011-2555A JP 2002-72082 A

  In an optical system using a diffractive optical element, if the effect of reducing the chromatic aberration of the element itself and the effect of an aspherical lens are strengthened too much, the refractive power on the diffractive surface of the diffractive optical part becomes too strong, and there is no need other than the designed diffraction order. Increased flare due to diffracted light is a problem. As a further improvement of the prior art, the present invention shares the effect of reducing the chromatic aberration of the diffractive optical element and the effect of an aspheric lens with the other optical elements in the diffractive optical element. Accordingly, an object of the present invention is to provide an optical system and an optical element in which the refractive power on the diffractive surface of the diffractive optical part is relaxed so that flare caused by unnecessary diffracted light can be suppressed.

  In order to achieve the above object, the present invention has a front group, an aperture stop, and a rear group in order from the object side, and the front group has a diffraction structure in which a plurality of diffraction gratings are stacked in order from the object side. An optical element having a focusing part and an optical element joined with a refractive optical part comprising a solid material having an anomalous partial dispersion characteristic to reduce the optical part and chromatic aberration, the refractive optical part being joined to the diffractive optical part The optical surface that is not formed is an aspherical shape facing the air.

  Further, the present invention is an optical element in which a diffractive optical part formed by laminating a plurality of diffraction gratings and a refractive optical part including a solid material having anomalous partial dispersion characteristics are joined, and the refractive optical part includes the diffractive optical part The optical surface which is not joined to the portion is an aspherical shape facing the air.

  According to the present invention, the effect of reducing the chromatic aberration of the diffractive optical element and the effect of an aspheric lens are shared by the other optical elements in the diffractive optical element, thereby reducing the refractive power on the diffractive surface of the diffractive optical part. Thus, flare caused by unnecessary diffracted light can be suppressed.

(A) is sectional drawing of the optical system which concerns on the 1st Embodiment of this invention, (b) is an enlarged view of the diffractive optical element which concerns on 1st Embodiment. It is an aberration diagram at the time of object infinity in the first embodiment. It is a figure explaining paraxial ray tracing in a 1st embodiment. It is a paraxial arrangement schematic diagram for explaining the optical action of a general telephoto type optical system. It is a graph which shows the refractive index characteristic ((theta) gF- (nu) d characteristic) of the solid material which forms the refractive optical part which concerns on 1st Embodiment. It is a simplified diagram for explaining flare of unnecessary diffraction orders due to light outside the field of view. It is a graph showing the diffraction efficiency of the unnecessary diffraction order of a diffractive optical element. It is a graph showing the amount of aspherical components of an element part in a 1st embodiment. It is a graph which shows the diffraction efficiency characteristic in the design order +/- 1st order of a diffraction optical part. It is a graph which shows the refractive index characteristic (nd- (nu) d characteristic) of the material which comprises a diffractive optical part. It is a graph which shows the refractive index characteristic ((theta) gF- (nu) d characteristic) of the material which comprises a diffractive optical part. It is a graph which shows the refractive index wavelength dependence characteristic of the material which comprises a diffractive optical part. (A), (b), (c) is a figure explaining the preparation methods of the diffractive optical element based on this invention. (A) is sectional drawing of the optical system which concerns on the 2nd Embodiment of this invention, (b) is an enlarged view of the diffractive optical element which concerns on 2nd Embodiment. FIG. 10 is an aberration diagram at object infinity in the second embodiment.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<< First Embodiment >>
(Overall configuration of optical system)
The optical system of this embodiment is a super telephoto lens (focal length 400 mm, Fno 4.0), and FIG. 1A shows a lens cross-sectional view at an object distance of infinity. In FIG. 1A, LF is a front group having positive refractive power, LR is a rear group having positive refractive power, and S is an aperture stop. The aperture stop S is disposed between the front group LF and the rear group LR. O represents an optical axis, IP represents an image plane, and G represents a glass block such as a crystal low-pass filter or an infrared cut filter. The diffractive optical element Ldam according to the present embodiment is the second lens counted from the object side, and details of the element configuration will be described later.

  Further, focusing from an infinite object point to a closest object point is performed by moving the cemented lens Lfo closest to the image plane in the front group LF to the image plane side as a focusing portion. Further, by moving the lens unit (LIS) in the rear group LR in a direction substantially perpendicular to the optical axis O, it is possible to correct image blur due to camera shake or the like.

(Configuration of diffractive optical element)
Here, the element configuration of the diffractive optical element Ldam will be described with reference to FIG. FIG. 1B is an explanatory diagram in which the second lens Ldam counted from the object side in the optical system according to the present embodiment is enlarged to an appropriate scale for easy explanation. In particular, the thicknesses ddoe and danm on the optical axis of the diffractive optical unit 102 and the refractive optical unit 103 and the shape of the diffraction grating formed in the direction perpendicular to the optical axis O of the diffractive optical unit are considerably deformed. Δdb is the amount of aspherical component, P is the grating pitch on the diffraction surface of the diffractive optical section 102, and dh is the grating thickness.

  As shown in FIG. 1B, the element configuration of the diffractive optical element Ldam (101) is that the glass substrate 104, the diffractive optical part 102 having the diffractive surface, and the anomalous partial dispersion are sequentially arranged from the object side (the left side of the paper in FIG. 1B). The refractive optical unit 103 is made of a solid material, and each part is tightly bonded. The optical surface of the refractive optical unit 103 on the image plane side (the right side in FIG. 1B) faces air and has an aspherical shape.

(Aberration diagram of optical system)
FIG. 2 shows aberration diagrams of the optical system according to this embodiment at an infinite object distance. In the spherical aberration of FIG. 2, the solid line represents the d line, the two-dot chain line represents the g line, the one-dot chain line represents the C line, and the dotted line represents the F line. Astigmatism, the solid line is the d-line sagittal ray (ΔS), the dotted line is the d-line meridional ray (ΔM), the dashed-dotted line is the g-line sagittal ray (ΔSg), and the two-dot chain line is the g-line meridional ray. (ΔMg) is shown respectively. Furthermore, in lateral chromatic aberration, the two-dot chain line represents the g line, the one-dot chain line represents the C line, and the dotted line represents the F line.

(Lens configuration of optical system)
Here, the lens configuration of the present embodiment will be described in more detail. In this embodiment, the diffractive optical element 101 is disposed on the object side from the aperture stop S at a position that satisfies the following conditional expression (1). The diffractive optical element 101 has a diffractive optical unit 102 that satisfies the following conditional expression (2) and is formed by closely laminating two diffraction gratings made of two types of materials having different dispersions on each other's grating surface. ing.

  A refractive action made of a solid material that is different from the material constituting the diffractive optical part 102 and satisfies the following conditional expressions (3) and (4) on either one of the light incident / exit surfaces of the diffractive optical part 102 The refractive optical unit 103 having The optical surface 105 that is not tightly bonded to the diffractive optical portion of the refractive optical portion 103 is characterized in that it faces air and has an aspherical shape.

0.5 <| h | <1.0 ----------- (1)
0.01 <| f / fdoe | <0.1 --------- (2)
When ΔθgF = θgF − (− 1.665 × 10 ⁻⁷ × νd ³ + 5.213 × 10 ⁻⁵ × νd ² −5.656 × 10 ⁻³ × νd + 0.7278) 0.02 <ΔθgF <0. 25 ----------------- (3)
10 <νd <40 ----------------- (4)
Here, h is the height from the optical axis when passing through the diffraction surface of the diffractive optical element on the axial paraxial ray incident at a height 1 from the optical axis parallel to the optical axis of the optical system. f is the focal length of the entire optical system, and fdoe is the focal length on the diffractive surface of the diffractive optical part of the diffractive optical element. νd is the Abbe number of the solid material represented by νd = (nd−1) / (nF−nC), and θgF is the partial dispersion ratio of the solid material represented by θgF = (ng−nF) / (nF−nC). , ΔθgF represents the abnormal partial dispersion ratio expressed by the above equation.

  For convenience of explanation, first, a solid material that satisfies the conditional expressions (3) and (4) used for the refractive optical unit 103 of the diffractive optical element will be described. Conditional expressions (3) and (4) are conditional expressions that define the range of material properties of the solid material used for the refractive optical unit 103 of the diffractive optical element. FIG. 5 is a graph showing the relationship, with the Abbe number νd on the horizontal axis and θgF on the vertical axis. From FIG. 5, the solid material within the range satisfying the conditional expressions (3) and (4) has higher dispersion (small value of νd) and high partial dispersion (large value of θgF) compared to general glass. I understand that there is.

  Incidentally, the material property values of the solid material used in this embodiment are (nd, νd, θgF) = (1.636, 22.7, 0.69), and are plotted in FIG. However, as long as it is in the range of conditional expressions (3) and (4), it is not limited to this. Here, the reason why the material characteristics of the solid material are preferably highly dispersed (value of νd is small) and highly partially dispersed (value of θgF is large) will be described. In general, when the refractive power change of the surface of the refractive lens is Δψ, the Abbe number is ν, the height from the optical axis through which the axial paraxial ray and the pupil paraxial ray pass through the lens surface are h and hb, respectively. The axial chromatic aberration coefficient change ΔL and the magnification chromatic aberration coefficient change ΔT on the lens surface can be expressed as follows.

ΔL = h ² * Δψ / ν ――――――――― (a)
ΔT = h * hb * Δψ / ν ――――――――― (b)
As is apparent from the equations (a) and (b), the change in each aberration coefficient with respect to the change in the refractive power of the lens surface becomes larger as the absolute value of the Abbe number is smaller (that is, higher dispersion). Therefore, if a high dispersion material having a small absolute value of the Abbe number is used, the amount of change in refractive power for obtaining a necessary chromatic aberration correction amount can be reduced. This means that chromatic aberration can be controlled without greatly affecting spherical aberration, coma aberration, astigmatism, etc. in terms of aberration theory, and the independence of chromatic aberration correction is enhanced.

  On the other hand, when a low dispersion material is used, the amount of change in refractive power for obtaining the necessary amount of chromatic aberration correction increases, and as a result, various aberrations such as spherical aberration change greatly, reducing the independence of chromatic aberration correction. Become. Therefore, it is important for aberration correction that at least one of the lenses constituting the optical system is a refractive lens formed of a high dispersion material.

  Next, based on the fact that the dispersion is high, the effect of an optical material having a high partial dispersion ratio on aberration correction of the optical system will be described. As is well known, in the wavelength dependence characteristics (dispersion characteristics) of the refractive index of an optical material, the Abbe number represents the overall slope of the dispersion characteristic curve, and the partial dispersion ratio represents the degree of bending of the dispersion characteristic curve. . In general, optical materials have a refractive index on the short wavelength side higher than that on the long wavelength side (Abbe number is a positive value), and the dispersion characteristic curve is convex downward (a partial dispersion ratio is a positive value). The shorter the wavelength, the greater the change in refractive index with respect to the change in wavelength.

  And, the higher dispersion optical material having a smaller Abbe number has a higher partial dispersion ratio, and the dispersion characteristic curve tends to be more convex downward. In an optical material with a large partial dispersion ratio, the wavelength-dependent characteristic curve of the chromatic aberration coefficient of the lens surface using the material shows a larger curve on the short wavelength side than when an optical material with a small partial dispersion ratio is used. At this time, when the refractive power of the lens surface is changed to control chromatic aberration, the overall inclination of the chromatic aberration coefficient wavelength characteristic curve changes with the position of the design reference wavelength as the rotation center.

  As for this change, the material with a large partial dispersion ratio has a particularly large movement on the short wavelength side as compared with a material with a small partial dispersion ratio, and the overall inclination changes while greatly changing the amount of bending. For this reason, even if the glass of another refractive system is changed, it becomes difficult to make a configuration that cancels both the overall inclination and the curve in the wavelength dependence characteristic curve of the chromatic aberration coefficient, and chromatic aberration can be corrected over the entire wavelength range. It will disappear. Therefore, it is possible to correct chromatic aberration by appropriately using a material having a low partial dispersion ratio that can reduce the bending component on the short wavelength side compared to ordinary glass materials.

  However, from the viewpoint of reducing chromatic aberration on the short wavelength side, it is considered that the same thing can be achieved if a material with a high partial dispersion ratio is used with a refractive power opposite to that of a material with a low partial dispersion ratio. That is, by using a material with a high partial dispersion ratio with opposite refractive power, the bending component and the inclination of the entire chromatic aberration wavelength dependence characteristic curve generated in other refractive optical systems in the optical system can be canceled independently and simultaneously. Good.

  Summarizing the above, it can be explained that it is effective in correcting the chromatic aberration of the optical system that the material properties of the solid material are high dispersion (value of νd is small) and high partial dispersion (value of θgF is large). .

  In conditional expression (3), if the upper limit is exceeded, there will be no material having such characteristics, which is not preferable. On the other hand, if the lower limit value is exceeded, the material properties of the solid material will be insufficient for correcting the desired chromatic aberration, which is not preferable.

  More preferably, the range of the material characteristics of the solid material is as follows, because chromatic aberration correction and refractive power of the diffractive surface of the diffractive optical element described later can be weakened.

0.02 <ΔθgF <0.15 ----------- (3-a)
15 <νd <30 ----------------- (4-a)
In the above, the range of the material characteristic of the solid material used for the refractive optical part of the diffractive optical element of the optical system has been described.

(Diffraction optical element and axial and lateral chromatic aberration)
Next, the conditions required for the diffractive optical element required when correcting the chromatic aberration of the telephoto type optical system that is the subject of the present invention will be described from the viewpoints of axial and magnification chromatic aberration coefficients. FIG. 4 is a schematic diagram of a paraxial arrangement for explaining the operation of a general telephoto type optical system. Gp and Gn are a front group of positive refractive power and a rear group of negative refractive power, respectively, constituting the telephoto type optical system, and Gdam is a diffractive optical element. In order to simplify the problem, it is assumed that the lenses constituting Gp and Gn are all thin single lenses and are arranged on the optical axis within Gp and Gn with a lens interval of 0, respectively.

Further, Gdam is also a thin single lens and is arranged on the optical axis O with a lens interval of 0. Q is the axial paraxial ray, R is the pupil paraxial ray, and P is the intersection of the pupil paraxial ray and the optical axis O.
First, consider an optical system before introducing the diffractive optical element Gdam. Formulas of axial chromatic aberration coefficient (L) and magnification chromatic aberration coefficient (T) are established for Gp and Gn.

It becomes. However,
ν _Gpi (λ) = {N _Gpi (λ ₀ ) −1} / {N _Gpi (λ) −N _Gpi (λ ₀ )}
ν _Gni (λ) = {N _Gnj (λ ₀ ) −1} / {N _Gnj (λ) −N _Gnj (λ ₀ )}
here,
φ _Gpi : refractive power of each thin single lens constituting the front group Gp φ _Gni : refractive power of each thin single lens constituting the rear group Gn ν _Gpi : Abbe number of each thin single lens constituting the front group Gp _Gni : Abbe number h _Gp of each thin single lens constituting the rear group Gn: _{Height of} the axial paraxial ray incident on the front group Gp h _Gn : Height of the axial paraxial ray incident on the rear group Gn H _Gp : Height of pupil paraxial ray incident on front group Gp H _Gn : Height of pupil paraxial ray incident on rear group Gn N _Gpi : Refractive index N of each thin single lens constituting front group Gp _Gnj : Refractive index λ of each thin single lens constituting the rear group Gn: Arbitrary wavelength λ ₀ : Design wavelength.

  In the normal telephoto type optical system, in the longitudinal chromatic aberration coefficient wavelength dependence characteristic of the equation (c), the overall inclination of the longitudinal chromatic aberration coefficient wavelength dependence characteristic of the front group Gp in the first term is negative and compared with the above. Show a strong tendency to convex. On the other hand, the longitudinal chromatic aberration coefficient wavelength dependence characteristic of the rear group Gn of the second term has a positive overall inclination and a downward convex tendency. As a result, the characteristics of the front group Gp remain a little as a whole system, The slope of the chromatic aberration coefficient is negative, and the upwardly convex axial chromatic aberration coefficient wavelength dependence characteristic is shown.

(Code of diffractive optical element and introduction position)
(Correction of longitudinal chromatic aberration)
Next, correction of axial chromatic aberration will be described when considering the sign and introduction position of the diffractive optical element Gdam for correcting axial chromatic aberration from this state. The diffractive optical element Gdam to be introduced is composed of the components of the diffractive optical part Gdoe including the diffractive surface and the refractive optical part Ganm having the high partial dispersion characteristic.
L _Gdam (λ) = h _Gdoe ² (λ ₀ ) φ _Gdoe (λ ₀ ) / ν _Gdoe (λ) + h _Ganm ² (λ ₀ ) φ _Ganm (λ ₀ ) / ν _Ganm (λ) −−−−−− (E)
And
However,
ν _Gdoe (λ) = − 3.453
[ _{nu] Ganm} ([lambda]) = { _NGanm ([lambda] ₀ ) -1} / { _NGanm ([lambda])- _NGanm ([lambda] ₀ )}.
It is.

In equation (e), the first term on the right side represents the axial chromatic aberration coefficient of the diffractive optical part Gdoe of the diffractive optical element, and the second term on the right side represents the axial chromatic aberration coefficient of the refractive optical part Ganm of the diffractive optical element. Yes. First, from the equation (e), the diffractive optical part (the first term on the right side) of the diffractive optical element has a constant of ν _Gdoe (λ) = − 3.453, and thus becomes a constant straight line independent of the wavelength. Further, the inclination is opposite because the Abbe number of ordinary glass is positive and takes a negative value.

Therefore, the axial chromatic aberration coefficient wavelength dependence characteristic of the diffractive optical part Gdoe is φ _Gdoe (λ ₀ )> 0, the whole inclination is a positive straight line, φ _Gdoe (λ ₀ ) <0, and the whole inclination is negative. It becomes a straight line. Therefore, in the optical system excluding the diffractive optical element of the formula (c), φ _Gdoe (λ ₀ )> 0 is necessary to cancel the entire inclination component of the axial chromatic aberration coefficient wavelength dependence characteristic curve. On the other hand, in the refractive optical part Ganm (second term on the right side) of the diffractive optical element, the inclination of the dispersion characteristic N _Ganm (λ) of the refractive optical part Ganm and the tendency of the bending component are directly reflected in 1 / ν _Ganm (λ). The

Therefore, the on-axis chromatic aberration coefficient wavelength-dependent characteristic of the refractive optical part Ganm is φ _Ganm (λ ₀ )> 0, the overall inclination is negative, and the curve is convex downward, and φ _Ganm (λ ₀ ) <0, The overall slope is positive and the curve is convex upward. Therefore, in the optical system excluding the diffractive optical element of the above formula (c), in order to cancel the entire inclination component of the axial chromatic aberration coefficient wavelength-dependent characteristic curve, it is usually necessary to have φ _Gdoe (λ ₀ ) <0. Become. At this time, the bending component is promoted, but the positive lens constituting the front lens group LF is close to high dispersion (dispersion curve is large) and the negative lens is low dispersion (dispersion curve is small). A glass material may be used.

This can be solved by making the longitudinal chromatic aberration wavelength-dependent characteristic curve of the entire system excluding the diffractive optical element a convex curve with a large negative slope. The overall inclination greatly deviated by changing the glass material may be obtained by displacing the refractive power φ _Gdam (λ ₀ ) of the diffractive optical element Gdam in the negative direction again. As a result, an axial chromatic aberration coefficient wavelength-dependent characteristic that is well corrected by both the entire inclination component and the bending component is obtained.

Regarding the introduction position of the diffractive optical element Gdam, since h _Gp ² >> h _Gn ² than h _Gp > h _Gn , Gdam has a relatively small refractive power, and the displacement amount of the refractive power when correcting chromatic aberration is relatively small. The front group LF that can be small, especially the part located on the object side is better. As a result, the independent correction of chromatic aberration is enhanced, and an axial chromatic aberration coefficient wavelength-dependent characteristic curve having relatively high linearity can be given to Gdam. That is, it is possible to reduce the curved component of the convex on the axial chromatic aberration coefficient wavelength-dependent characteristic curve promoted by Gdam including the time of chromatic aberration correction.

  Further, it is possible to relatively easily make a downward convex curve without giving a large negative slope to the axial chromatic aberration wavelength dependence characteristic curve of the entire system excluding Gdam. The correction of axial chromatic aberration has been described above.

(Correction of lateral chromatic aberration)
Next, correction of lateral chromatic aberration will be described. In the normal telephoto type optical system, in the magnification chromatic aberration coefficient wavelength dependence characteristic of the equation (d), the overall chromatic aberration coefficient wavelength dependence characteristic of the front group LF in the first term has a positive overall inclination and is relatively strong below. Shows a convex tendency. On the other hand, the chromatic aberration coefficient wavelength dependence characteristic of the rear group LR of the second term has a negative overall inclination and a tendency of upward convexity. As a result, the characteristics of the front group LF remain slightly in the entire system, It shows positive wavelength chromatic aberration coefficient wavelength dependence with a positive slope. The magnification chromatic aberration coefficient of Gdam introduced to the front lens group LF for the above-mentioned axial chromatic aberration correction is
T _Gdam (λ) = h _Gp (λ ₀ ) H _Gp (λ ₀ ) * {φ _Gdoe (λ ₀ ) / ν _Gdoe (λ) + φ _Ganm (λ ₀ ) / ν _Ganm (λ)} −−−− ---- (f)
It is. In the formula (f), the first term on the right side represents the lateral chromatic aberration coefficient of the diffractive optical unit 102 of the diffractive optical element, and the second term on the right side represents the lateral chromatic aberration coefficient of the refractive optical unit 103 of the diffractive optical element. From the explanation about the axial chromatic aberration coefficient, h _Gp (λ ₀ )> 0, H _Gp (λ ₀ ) <0, and φ _Gdoe (λ ₀ ) / ν _Gdoe (λ) <0, φ _Ganm (0 Since [lambda] ₀ ) / [ _{nu] Ganm} ([lambda]) <0, the overall inclination is negative and the curve is convex downward. As a result, the entire inclination component of the magnification chromatic aberration coefficient wavelength-dependent characteristic curve of equation (d) can be canceled.

At this time, the bending component is promoted, but when correcting the above-mentioned axial chromatic aberration, the positive lens constituting the front lens group LF is close to high dispersion (a large curve of dispersion characteristics), and the negative lens is low dispersion. What is necessary is just to use a glass material that is close (small bend in dispersion characteristics). As a result, the chromatic aberration wavelength-dependent characteristic curve of the entire system excluding the diffractive optical element Gdam has a large positive slope and is a convex curve, and the bending component is canceled at the same time. The overall inclination greatly deviated by changing the glass material is also corrected by displacing the refractive power φ _GIdam (λ ₀ ) of _Gdam in the negative direction again when correcting the above-described axial chromatic aberration.

  As described above, it has been described that the axial chromatic aberration and the lateral chromatic aberration can be corrected simultaneously by giving an appropriate refractive power to the diffractive optical element Gdam and introducing it into the front group Gp.

(Requirements for diffraction optics)
Based on the above description, the remaining conditional expressions (1) and (2) will be described. The conditional expression (1) is a conditional expression that defines the location of the diffractive optical element Ldam. Here, in order to make the explanation of the conditional expression (1) easy to understand, it will be explained with reference to FIG. In FIG. 3, the horizontal axis is the lens surface number, the vertical axis is the paraxial tracking value, and the on-axis paraxial ray h and the pupil paraxial ray hb are shown together. As shown in FIG. 3, in the super telephoto lens of this embodiment, the axial paraxial ray h gradually decreases as the lens surface number increases (the ray goes from the object side to the image plane side). I understand.

  On the other hand, as the pupil paraxial ray becomes 0 at the entrance pupil position (the aperture stop position of lens surface number 15 in the figure), the lens surface number increases (the light ray moves from the object side to the image surface side), It changes from a negative value to a positive value. From the above description, in the present embodiment, the diffractive optical element is located at the second lens position (lens surface number 4) counted from the object side where the axial paraxial ray h and the pupil paraxial ray hb pass are relatively high. By arranging Ldam, axial and lateral chromatic aberration are simultaneously corrected.

  In the conditional expression (1), if the upper limit value is exceeded, the position where the diffractive optical element Ldam is arranged becomes the lens surface closest to the object side, which is easily affected by dust, external heat, etc., which is preferable in terms of environmental resistance. Absent. Further, as the arrangement of the diffractive optical element is closer to the object side, strong light such as sunlight outside the shooting angle of view is likely to directly hit the diffractive optical element, and flare generated thereby leads to image quality deterioration, which is not preferable ( This will be described later). On the other hand, if the lower limit value is exceeded, the location of the diffractive optical element Ldam is too close to the vicinity of the aperture stop.

  More preferably, it is within the range of the following conditional expression because a chromatic aberration correction effect can be obtained more.

0.6 <| h | <1.0 ----------------- (1-a)
Next, the conditional expression (2) is a conditional expression that defines the relationship between the focal length of the diffraction surface of the diffractive optical part of the diffractive optical element and the focal length of the entire optical system. Here, the focal length fdoe on the diffractive surface of the diffractive optical unit is the phase coefficient C1, the design diffraction order m, the design wavelength λ0, and the arbitrary wavelength λ in the optical system, and fdoe = −1 / (2 * m *). C1 * λ / λ0). Exceeding the upper limit value of conditional expression (2) is not preferable because the refractive power of the diffractive surface becomes too strong, the grating pitch becomes fine, and flare due to unnecessary diffracted light tends to deteriorate as described later.

  On the other hand, if the lower limit is exceeded, the refractive power of the diffractive surface becomes too weak, and the desired effect of chromatic aberration cannot be obtained. The diffractive optical element of the present embodiment is composed of a diffractive optical part having a diffractive surface and a refracting optical part made of a solid material having anomalous partial dispersion characteristics, so that the refractive power of the diffractive surface becomes too weak. This is not preferable because the refractive optical portion becomes thick.

(Refractive power and flare of diffractive optics)
Here, the relationship between the refractive power of the diffractive surface of the diffractive optical unit and the flare, particularly the flare generated when strong light such as sunlight outside the shooting angle of view directly strikes the diffractive optical element will be described. As described above, the refractive power (focal length) of the diffractive surface is calculated by the phase coefficient C1 from fdoe = −1 / (2 * m * C1 * λ / λ0). The phase shape ψ of the diffractive surface has a diffraction order m of the diffracted light, a design wavelength λ 0, a height perpendicular to the optical axis r, and a phase coefficient Ci (i = 1, 2, 3...). Is expressed by the following equation (g).

ψ (r, m) = (2π / mλ0) * (C1 * r ^ 2 + C2 * r ^ 4 + C3 * r ^ 6 +...) −−−−−−− (g)
From the equation (g), the position where the phase is an integral multiple of the design wavelength in the direction perpendicular to the optical axis on the diffraction surface, that is, the grating pitch of each diffraction grating is calculated. Generally, as the refractive power of the diffraction surface increases, the grating pitch of the diffraction grating tends to become finer.

  Next, flare generated when strong light such as sunlight outside the shooting angle of view directly strikes the diffractive optical element will be described with reference to FIG. FIG. 6 is a schematic diagram for explaining how flare occurs in the optical system having the same specifications as the present embodiment. In FIG. 6, the cemented lens closest to the object is a diffractive optical element, S is an aperture stop, and IP is an image plane. When strong light such as sunlight enters the diffractive optical element at an angle ω from outside the field of view, light with a diffraction order other than the designed order (unnecessary diffraction order light) is generated on the diffractive surface of the diffractive optical element. The diffraction order light passes through the subsequent lens group and the aperture stop S, and finally reaches the image plane IP.

  Unnecessary diffraction order light reaching the image plane IP becomes flare. The unnecessary diffraction order light has different energy depending on each diffraction order. The ratio of this energy is the diffraction efficiency, which is defined by the ratio of the amount of diffracted light at each order with respect to the total amount of transmitted light. Usually, the primary light is used for the photographing light, and it is desirable that the diffraction efficiency of the primary light approaches 100% because unnecessary diffraction order light is reduced.

(Flare and diffraction efficiency)
Next, the diffraction efficiency assuming the flare occurrence state of FIG. 6 will be described with reference to FIG. First, the assumed configuration of the diffractive optical element is premised on a diffractive optical element (a close-contact two-layer DOE) in which two diffraction gratings made of different materials are in close contact with each other on their diffractive surfaces. The configuration of this diffractive optical element will be described later. Here, an acrylic resin material (nd = 1.522, νd = 51.3) is used for the first diffraction grating, and a material obtained by mixing ITO fine particles with fluorine resin for the second diffraction grating (nd = 1. 480, νd = 21.3).

  At this time, the common grating thickness of the first and second diffraction gratings is 13.9 μm. Considering the diffraction efficiency when the diffractive optical element is incident at an incident angle ω = 10 deg, assuming light outside the field of view as shown in FIG. Note that strict coupled wave analysis (RCWA: Regulated Coupled Wave Analysis) was used for the calculation of the diffraction efficiency efficiency. FIG. 7 shows the grating pitch dependence of the diffraction efficiency of the close-contact two-layer DOE mentioned as an example.

  In FIG. 7, the horizontal axis represents the diffraction angle (deg) corresponding to the diffraction order light emitted from the diffractive optical element, the vertical axis represents the diffraction efficiency (%), and the grating pitch is changed to 80, 100, and 150 μm. The result is shown. Assuming a super-telephoto optical system as shown in FIG. 6, since it is known that unnecessary diffracted light having an exit diffraction angle near 0 deg reaches the image plane, the diffraction efficiencies of the portions are compared. FIG. 7 shows that the diffraction efficiency of unnecessary diffracted light near 0 deg increases as the grating pitch becomes finer.

  Although the results of other grating pitches are not shown in FIG. 7, the finer the grating pitch, the more unnecessary diffraction light in the vicinity of the exit diffraction angle 0 deg that reaches the image plane tends to increase. That is, as the refractive power of the diffractive surface of the diffractive optical unit is increased, unnecessary diffracted light generated by the strong light such as sunlight outside the shooting angle of view directly hits the diffractive optical element flare and reaches the image surface. Is meant to do.

  Therefore, in the diffractive optical element having a diffractive optical part having a diffractive surface and a refractive optical part made of a solid material having anomalous partial dispersion characteristics, the refractive power of the diffractive optical part of the diffractive optical part is reduced so that flare can be reduced. It is preferable to make it. Preferably, the conditional expression (2) is more effective in suppressing flare when it is in the following range.

0.015 <| f / fdoe | <0.05 ------------ (2-a)
(Aspherical shape of refractive optical part)
In this embodiment, the refractive power of the diffractive optical unit is reduced by sharing the effect of reducing the chromatic aberration inherent in the diffractive optical element and the effect of an aspheric lens with the other optical elements in the diffractive optical element of the present invention. To suppress flare caused by unnecessary diffracted light. The former is shared by a refractive optical unit made of a solid material having the above-described anomalous partial dispersion characteristics. The latter is shared by the aspherical shape of the predetermined optical surface described below.

  That is, the present embodiment is characterized in that, after satisfying the conditional expressions (1) to (4) described above, the following conditional expression (5) is also satisfied. First, conditional expression (5) is a conditional expression that defines the aspherical shape of the optical surface 105 facing the air of the refractive optical portion of the diffractive optical element. R is a paraxial radius of curvature, r is a height from the optical axis in a direction perpendicular to the optical axis, k is a conic constant, B, C, D, E... Are aspherical coefficients of respective orders.

Δdb = ((1 / R) * r ² / (1 + √ (1− (1 + k) * (r / R) ² )) + B * r ⁴ + C * r ⁶ + D * r ⁸ + E * r ¹⁰ +. When ((1 / R) * r ² / (1 + √ (1− (r / R) ² ))), the following expression (5) is satisfied.

0.01 <| Δdb_max / danm | <0.50 ---------- (5)
Here, Δdb is the amount of the aspherical component defined by the above equation, and Δdb_max is the maximum aspherical surface within the effective radius of the light beam passing through the optical surface in the optical surface provided with the aspherical shape defined by Δdb. Indicates the amount of ingredients. Further, danm represents the thickness on the optical axis of the refractive optical part of the diffractive optical element. This relationship is visualized and graphed in FIG. 1 (b) and FIG. As shown in FIG. 1B, in the present embodiment, an aspherical component Δdb is added to the optical surface 105 closest to the image plane (right side in the drawing) in a direction in which the curvature becomes tighter.

  FIG. 8 is a graph showing the relationship, where the horizontal axis represents the height in the radial direction (radius (mm) perpendicular to the optical axis), and the vertical axis represents the amount of aspheric component (mm). ing. The amount of aspherical component on the vertical axis is the amount defined by the expression Δdb. The sign is + and the aspherical component is added to the standard curvature, and the sign is – and the aspherical component is removed. Direction. As can be seen from FIG. 8, in the present embodiment, the amount of the aspherical component increases in the direction in which the aspherical component is added to the reference curvature and as the radial height increases. The maximum value Δdb_max within the effective beam diameter of the value is Δdb_max = 0.248 (mm) at a radial height of about 40 mm.

  In conditional expression (5), if the upper limit is exceeded, the amount of aspherical components in the radial direction tends to increase further, which is not preferable from the viewpoint of environmental resistance such as water absorption. On the other hand, if the lower limit of conditional expression (5) is exceeded, the optical performance of various aberrations including the desired spherical aberration cannot be obtained, and it is not preferable because the entire optical system cannot be reduced in size and weight. From the viewpoint of achieving both compactness and light weight of the entire optical system and element molding, it is desirable that the range of the following conditional expressions be satisfied.

0.05 <| Δdb_max / danm | <0.20 --------- (5-a)
Conditional expression (6) is a conditional expression that defines the relationship between the focal length of the refractive optical section of the diffractive optical element and the focal length of the entire optical system. In the optical system according to this embodiment, when the focal length of the refractive optical unit 103 of the diffractive optical element is set to fanm, the following conditional expression (6) is satisfied.

0.2 <| fanm / f | <5.0 --------------- (6)
Exceeding the upper limit value of conditional expression (6) is not preferable because the refractive power of the refractive optical unit 103 becomes too weak and the desired optical performance, particularly the chromatic aberration correction effect cannot be obtained. Further, considering the entire diffractive optical element, the refractive power of the diffractive optical unit 102 cannot be weakened, which is not preferable from the viewpoint of reducing flare. On the other hand, when the lower limit of conditional expression (6) is exceeded, the refractive power of the refractive optical unit 103 becomes too strong, and the thickness of the refractive optical unit 103 itself tends to increase. This is not preferable. From the viewpoint of achieving both improvement in optical performance in the entire optical system and improvement in moldability of the element portion, it is more preferably within the range of the following conditional expression.

0.4 <| fanm / f | <4.0 -------------- (6-a)
As described above, by making the optical system using the diffractive optical element that satisfies the conditional expressions (1) to (6), the entire optical system is small and light, but various aberrations including chromatic aberration are good. An optical system corrected to be able to be achieved.

(Diffraction efficiency of diffractive optics in the entire wavelength range)
Next, the diffractive optical part of the diffractive optical element will be described using conditional expressions (7) to (15). In this embodiment, the wavelength region of light incident on the diffractive optical part 102 of the diffractive optical element, that is, the used wavelength region is the visible region, and the materials and the grating thicknesses constituting the first and second diffraction gratings 1 and 2 are as follows. The diffraction efficiency of the first-order diffracted light, which is the designed order, is selected to be high throughout the visible region. Here, the diffraction efficiency of the diffractive optical unit 102 of the diffractive optical element according to the present embodiment will be described.

  The principle is basically the same as that of a single-layer DOE (single-layer diffractive optical element) in which a plurality of diffraction gratings are arranged on one layer, and it functions as a single diffractive optical element through all layers. Think about it. The optical path length difference between the peaks and valleys of the diffraction grating formed at the boundary of the material constituting each layer is obtained, and this optical path length difference is added over the entire diffraction grating. Then, the dimension of the grating shape is determined so that the added optical path length difference is an integral multiple of the wavelength. Therefore, in the diffractive optical part 102 of the diffractive optical element shown in FIG. 1B, when the design wavelength is λ0, the conditions under which the diffraction efficiency of m-th order diffracted light is maximized are as follows.

± (n01−n02) * dh = m * λ0 −−−−− (h)
Here, n01 is a refractive index with respect to light having a wavelength λ0 of the material forming the first diffraction grating 1, and n02 is a refractive index with respect to light having a wavelength λ0 of the material forming the second diffraction grating 2. Further, dh is the grating thickness of the diffraction grating 1 (= 2).

  In FIG. 1B, the diffraction order of light diffracted downward from the 0th-order diffracted light (direction toward the optical axis O) is a positive diffraction order, and the diffraction order of light diffracted upward from the 0th-order diffracted light is a negative diffraction order. Then, the sign of addition / subtraction before the parenthesis on the left side in the above formula (h) is as follows. Focusing on the first diffraction grating 1 in the figure, a diffraction grating having a grating shape in which the grating thickness increases from top to bottom is positive, and conversely has a grating shape in which the grating thickness decreases from top to bottom. In the case of a diffraction grating, it is negative. In the configuration shown in FIG. 1B, the diffraction efficiency η (λ) at a wavelength λ other than the design wavelength λ0 can be expressed by the following equation.

η (λ) = sinc ² (π * (m− (± (n1 (λ) −n2 (λ)) * dh / λ)))
= Sinc ² (π * (m * φ (λ) / λ)) --------- (i)
φ (λ) = ± (n1 (λ) −n2 (λ)) * dh −−−−−−− (j)
Here, m is the diffraction order, n1 (λ) is the refractive index at the wavelength λ of the material forming the first diffraction grating 1, and n2 (λ) is the wavelength λ of the material forming the second diffraction grating 2. The refractive index dh is the common grating thickness of the first and second diffraction gratings 1 and 2. Moreover, it is a function represented by sinc ² (x) = (sin (x) / x) ² .

  Next, conditions for obtaining high diffraction efficiency in the diffractive optical part 102 of the diffractive optical element of this embodiment will be described. In order to obtain high diffraction efficiency over the entire use wavelength range, the value η (λ) expressed by the above equation (i) should be close to 1 for all use wavelengths. In other words, in order to increase the diffraction efficiency at the design order m, φ (λ) / λ in the above equation (i) should be close to m. For example, when the design order m is set to primary, φ (λ) / λ should be close to 1.

  Further, the optical optical path length difference φ (λ) obtained from the grating shape changes linearly in proportion to the wavelength λ from the above relationship, that is, the term on the right side of the above equation (j) has linearity. It becomes necessary. That is, the rate of change in the refractive index due to the wavelength of the material forming the second diffraction grating 2 with respect to the change in the refractive index due to the wavelength of the material forming the first diffraction grating 1 is a constant ratio in the entire used wavelength region. It will be necessary. This embodiment will be described as an example of a configuration that satisfies the above-described relationship.

  In the diffractive optical unit 102 shown in FIG. 1B, in the present embodiment, the first diffraction grating 1 is made of a material in which ZrO2 fine particles are mixed with acrylic resin (nd = 1.552, νd = 49.1, θgF). = 0.578). The second diffraction grating 2 is made of a material (nd = 1.495, νd = 18.7, θgF = 0.401) in which ITO fine particles are mixed with a fluorine resin. At this time, the common grating thickness of the first and second diffraction gratings is 10.2 μm. In the present embodiment, such a combination of materials is used. However, the present invention is not limited to this as long as conditional expressions (7) to (15) described below are satisfied.

  FIG. 9 also shows the wavelength characteristics of the diffraction efficiency of the first-order diffracted light, which is the designed order of the diffractive optical unit 102 in this embodiment, and the designed orders ± first-order light (zero-order light and second-order light). In FIG. 9, the horizontal axis represents wavelength (nm), the vertical axis represents diffraction efficiency (%), the left vertical scale for primary light, and the right vertical scale for 0 secondary light. Refer to As shown in FIG. 9, the diffraction efficiency of the first-order light is 99% or more in the entire visible range, and accordingly, the diffraction efficiency of the unnecessary diffraction orders (0th-order light and second-order light) is sufficiently suppressed to 0.2% or less. I understand that

  In addition, here, the diffraction efficiency of unnecessary-order light is targeted only for the 0th-order light and the second-order light that are the designed orders ± 1st. However, the diffraction order that is far from the design order has a ratio that contributes to flare. This is because there are few. That is, if the flare light of the 0th and 2nd diffraction orders is reduced, the influence of the flare light of the other diffraction orders can be similarly reduced.

  This is because, in part, the diffraction efficiency of a diffractive optical element designed to diffract light of a specific design order tends to decrease as it goes away from the design order. caused by. Furthermore, the diffraction order farther from the design order and the blur on the image plane become larger and become less noticeable as flare.

  Next, the reason why the first-order diffraction efficiency of the diffractive optical unit 102 according to this embodiment is as high as 99% or more will be described using conditional expressions (7) to (15). In the diffractive optical section 102, the wavelengths of the Fraunhofer line g-line, F-line, d-line, and C-line are λg, λF, λd, λC, and the g-line of the diffraction gratings 1 and 2, F-line, d-line, and C-line. The refractive indexes at the wavelengths are assumed to be n1g, n2g, n1F, n2F, n1d, n2d, n1C, and n2C. The grating thickness of the diffraction grating is dh, and the grating pitch of the diffraction grating is P.

  Further, the values obtained by dividing the difference in optical path length between the convex and concave portions of the diffraction grating by the respective wavelengths with respect to the M-th order diffracted light near the design order are as follows. That is, M (λg) = {dh × (n1g−n2g)} / λg, M (λF) = {dh × (n1F−n2F)} / λF, M (λd) = {dh × (n1d−n2d)} When satisfying / λd, M (λC) = {dh × (n1C−n2C)} / λC, the conditional expressions (7) to (15) are satisfied.

1.5 ≦ n1d ≦ 1.8 --------------------- (7)
40 ≦ νd1 = (n1d * 1) / (n1F * n1C) ≦ 80 ------- (8)
θ1gF = (n1g * n1F) / (n1F * n1C), and Δθ1gF = θ1gF − (− 1.665 × 10 ⁻⁷ × νd1 ³ + 5.213 × 10 ⁻⁵ × νd1 ² −5.656 × 10 ⁻³ × When νd1 + 0.7278), the following expression is satisfied.
-0.05 <Δθ1gF <0.05 ----------------- (9)
1.3 ≦ n2d ≦ 1.6 -------------------- (10)
5 ≦ νd2 = (n2d * 1) / (n2F * n2C) ≦ 30 ------- (11)
θ2gF = (n2g * n2F) / (n2F * n2C), and Δθ2gF = θ2gF − (− 1.665 × 10 ⁻⁷ × νd2 ³ + 5.213 × 10 ⁻⁵ × νd2 ² −5.656 × 10 ⁻³ × νd2 + 0.7278), the following expression is satisfied.

-0.50 <[Delta] [theta] 2gF <-0.02 --------------- (12)
0.01 <n1d * n2d <0.5 ------------------ (13)
0.01 <dh / p ≦ 0.1 -------------------- (14)
0.92 ≦ {M (λg) + M (λF) + M (λd) + M (λC)} / (4 * M) ≦ 1.08 −−−−−−−−−−−−−−− (15)
The conditional expressions (7) to (15) define the material range and conditions of the diffraction gratings 1 and 2 for increasing the diffraction efficiency of the designed order in the diffractive optical part 102 of the diffractive optical element of the present embodiment. This is the conditional expression.

  Here, in order to make it easy to imagine the relationship between the conditional expressions, description will be made with reference to FIGS. FIG. 10 shows the relationship between nd and νd, and FIG. 11 shows the relationship between θgF and νd. The vertical axes are nd and θgF, respectively, and the horizontal axes are all νd.

  First, with respect to FIG. 10 and the conditional expressions (7), (8), (10), and (11), the nd and νd of the materials of the diffraction gratings 1 and 2 for forming the diffractive optical part of this embodiment are shown. Defines the range. As can be seen from FIG. 10, the existence range of the first diffraction grating 1 is not different from that of ordinary glass, but the existence range of the second diffraction grating 2 is considerably small, that is, the dispersion is high. . If it exceeds the range of conditional expressions (7), (8), (10) and (11), the desired performance cannot be obtained with the configuration of the diffractive optical part of the present embodiment (adherent two-layer type). Absent.

  Next, with respect to FIG. 11 and the conditional expressions (8), (9), (11), and (12), each diffraction for achieving the diffractive optical part of this embodiment is the same as that described above with reference to FIG. The range of θgF and νd of the materials of the lattices 1 and 2 is defined. This relationship must be satisfied after satisfying the conditional expressions (7), (8), (10), and (11). As can be seen from FIG. 11, the existence range of the first diffraction grating 1 is slightly shifted upward from the ordinary glass, and the existence range of the second diffraction grating 2 is significantly displaced downward from the ordinary glass. . That is, it means that the material of the diffraction grating 1 has a slightly higher partial dispersion ratio than a general glass material, and the material of the diffraction grating 2 has a considerably lower partial dispersion ratio than a general glass material.

  Exceeding the range of conditional expressions (8), (9), (11), and (12) is not preferable because the diffraction efficiency particularly on the short wavelength side deteriorates. Further, the grating thickness of the diffractive optical part is increased, which leads to deterioration of diffraction efficiency particularly at oblique incidence, which is not preferable.

  Next, the refractive index wavelength characteristics of the materials of the diffraction gratings 1 and 2 used in the diffractive optical part of this embodiment are shown in FIG. In FIG. 12, the horizontal axis represents wavelength (nm) and the vertical axis represents refractive index. From FIG. 12, it can be seen that the material characteristic of the diffraction grating 1 is such that the slope of the curve becomes tighter as the wavelength is shorter, whereas the material characteristic of the diffraction grating 2 is that the change in refractive index with respect to the change in wavelength is almost constant. This clearly represents the relationship of the conditional expressions (8), (9), (11), and (12) in FIG. 12. The material of the diffraction grating 1 is a slightly high partial dispersion ratio, and the material of the diffraction grating 2 Indicates a fairly low partial dispersion ratio.

  In the description of the part of the expression (j), the second diffraction grating 2 is formed with respect to the change in the refractive index depending on the wavelength of the material forming the first diffraction grating 1 as a condition for setting the diffraction efficiency of the designed order to 100%. He stated that the rate of change of the refractive index with the wavelength of the material to be used is a constant ratio in the entire wavelength range. Since this embodiment substantially satisfies these conditions, high diffraction efficiency can be realized at a visible wavelength. Furthermore, in order to maintain the high diffraction efficiency of the diffractive optical part, it is desirable that the range is in the following conditional expression.

1.5 ≦ n1d ≦ 1.7 ------------------- (7-a)
40 ≦ νd1 = (n1d * 1) / (n1F * n1C) ≦ 65 −−−−− (8−a)
-0.03 <Δθ1gF <0.03 -------------- (9-a)
1.4 ≦ n2d ≦ 1.55 ------------------ (10-a)
10 ≦ νd2 = (n2d * 1) / (n2F * n2C) ≦ 25 −−−−− (11−a)
-0.30 <Δθ2gF <-0.05 ------------- (12-a)
0.01 <n1d * n2d <0.2 ---------------- (13-a)
In addition to satisfying the conditional expressions (7) to (13), the following conditional expressions (14) and (15) must be satisfied.

0.01 <dh / p ≦ 0.1 ------------------ (14)
0.92 ≦ {M (λg) + M (λF) + M (λd) + M (λC)} / (4 * M) ≦ 1.08 ----------------- ---------- (15)
Conditional expression (14) is a conditional expression that defines the range of the grating thickness with respect to the grating pitch p of the diffraction grating of the diffractive optical section according to the present embodiment. On the other hand, the conditional expression (15) is a conditional expression that defines the range of the diffraction efficiency wavelength characteristic of the diffractive optical part according to this embodiment. In conditional expression (14), if the upper limit value is exceeded, the grating thickness with respect to the grating pitch of the diffraction grating becomes too high, and deterioration of the desired diffraction efficiency, particularly diffraction efficiency at oblique incidence, is not preferable.

  Also, the grating pitch of the diffraction grating becomes too fine, making it difficult to manufacture. On the other hand, if the lower limit is exceeded, the grating pitch of the diffraction grating becomes too wide, the effect of diffraction action is diminished, and the desired optical performance cannot be obtained. Further, if the conditional expression (15) is outside the range of the conditional expression, it is not preferable because desired diffraction efficiency cannot be obtained in the entire visible wavelength range, and flare of unnecessary diffraction orders is generated. Furthermore, in order to maintain the high diffraction efficiency of the diffractive optical part, it is desirable that the range is in the following conditional expression.

0.01 <dh / p ≦ 0.08 ----------------- (14-a)
0.95 ≦ {M (λg) + M (λF) + M (λd) + M (λC)} / (4 * M) ≦ 1.05 ----------------- ---------- (15-a)
In order to obtain the material characteristics of the diffraction gratings 1 and 2 of the diffractive optical part, the following is performed. The material of the diffraction grating 1 is prepared by mixing and mixing inorganic fine particles of any one of Al, Zr, Y and their oxides, composites, and mixtures with an acrylic resin material so as to have desired material characteristics. On the other hand, the material of the diffraction grating 2 is adjusted by mixing inorganic fine particles of any one of ITO, Ti, Nr, Cr and their oxides, composites, and mixtures into a fluorine-based resin material so as to have desired material characteristics. realizable.

(Diffraction optical element manufacturing method)
Finally, an example of a method for manufacturing the diffractive optical element 101 of the present embodiment will be briefly described with reference to FIGS. First, the first diffraction grating 1 is formed by irradiation using a mold 202 on a glass substrate (FIG. 13A). At this time, a desired diffraction grating shape is formed on the interface between the mold 202 and the diffraction grating 1 (not shown in the figure). Next, the mold 202 is released from the diffraction grating 1 and the material of the diffraction grating 2 is dropped onto the diffraction grating 1 to produce the diffractive optical unit 102 using the mold 203 (FIG. 13B). At this time, the curvature of the boundary surface of the mold 203 is made substantially the same as the curvature of the diffraction surface of the diffraction grating 2.

  Finally, the mold 203 is released, and the refractive optical section 103 made of a solid material having desired material properties is irradiated and molded on the optical surface of the diffractive optical section 102 that is not on the glass substrate side using the mold 204 (FIG. 13 (c)). At this time, a desired aspheric shape is formed on the boundary surface of the refractive optical unit 103 of the mold 204. Finally, the mold 204 is released to complete the diffractive optical element of this embodiment.

  As described above, the present invention is not limited to those manufactured by this manufacturing method. As another manufacturing method of the diffractive optical element in the present embodiment, there is a method of directly forming a binary optics shape on the lens surface with a photoresist. In the embodiment described so far, as illustrated in FIG. 1B, the description has been made with the shape in which the diffractive portion is provided on the curved surface such as the convex surface or the concave surface of the lens. The same effect as can be obtained. In this embodiment, a diffractive optical element using so-called first-order diffracted light having a design order of 1 has been described. However, the design order is not limited to 1.

  Even if the diffracted light is different from the first order diffraction order such as the second order or the third order, the combined value of the optical path length differences in the diffraction gratings 1 and 2 is set to the desired design wavelength at the desired design order. If this is set, effects similar to those of the present embodiment can be obtained.

  As described above, the lens configuration and the element form of the diffractive optical element as shown in FIGS. 1A and 1B are satisfied so that the conditional expressions (1) to (15) are satisfied. Accordingly, it is possible to realize a photographing optical system in which various aberrations including chromatic aberration are corrected while the optical system itself is small and light. At that time, flare caused by unnecessary diffracted light other than the photographing light can be reduced.

  According to the present embodiment, the diffractive optical element in which the refractive optical unit made of a solid material having anomalous partial dispersion characteristics is disposed close to the object side from the aperture stop in the optical system is arranged with an appropriate refractive power to An optical system in which the system itself is small and light and various aberrations are corrected can be provided. At that time, flare caused by unnecessary diffracted light other than the photographing light can be reduced.

<< Second Embodiment >>
This embodiment is also a super telephoto lens (focal length 400 mm, Fno 4.0), and FIG. 14A shows a lens cross-sectional view at an infinite object distance. The element configuration of the diffractive optical element Ldam is shown in FIG. Since the basic configuration and the idea of the invention are the same as those of the first embodiment, only different portions will be described below.

  The diffractive optical element Ldam according to the present embodiment is a lens closest to the object side, and this is another example based on the simultaneous solution of the on-axis and lateral chromatic aberration of the telephoto type optical system described in the first embodiment. Show. This is the most effective position for correcting both chromatic aberrations. Also, from FIG. 14B, the different part in the element configuration of the diffractive optical element Ldam is the aspherical component amount in the radial direction, and the maximum value Δdb_max within the effective ray diameter is about 47 mm in the radial direction. Δdb_max = + 0.221 (mm). An aberration diagram of this embodiment is shown in FIG. FIG. 15 shows that each aberration is well corrected.

  As described above, if the lens configuration and the element form of the diffractive optical element as shown in FIGS. 14 and 15 are satisfied and the conditional expressions (1) to (15) are satisfied, the optical system itself is small and lightweight. However, it is possible to realize a photographing optical system in which various aberrations including chromatic aberration are corrected. At that time, flare caused by unnecessary diffracted light other than the photographing light can be reduced.

(Numerical embodiment)
Next, numerical embodiments will be described. In each numerical embodiment, ri is the radius of curvature of the i-th lens surface from the object side, di is the distance between the upper surfaces of the axes in the i-th reference state from the object side, and ndi and νdi are d-lines of the i-th optical member. Represents the refractive index and the Abbe number, respectively. BF is back focus. Further, the phase shape ψ of the diffractive optical surface of each embodiment is that the diffraction order of the diffracted light is m, the design wavelength is λ0, the height in the direction perpendicular to the optical axis is r, and the phase coefficient is Ci (i = 1, 2). 3)), it is expressed by the following equation.

ψ (r, m) = (2π / mλ0) * (C1 * r ^ 2 + C2 * r ^ 4 + C3 * r ^ 6 +...)
Further, in the aspherical shape, X is the amount of displacement from the surface vertex in the optical axis direction, r is the height from the optical axis in the direction perpendicular to the optical axis, R is the paraxial radius of curvature, k is the conic constant, B, When C, D, E... Are the aspheric coefficients of the respective orders, they are expressed by the following equations.

(Numerical example 1)
Unit mm
Surface data surface number rd nd vd Effective diameter
1 103.443 12.58 1.48749 70.2 95.25
2 682.412 15.00 94.39
3 64.475 9.01 1.48749 70.2 82.35
4 107.769 0.03 1.55184 49.1 80.78
5 (Diffraction) 107.739 0.03 1.49515 18.7 80.75
6 107.709 2.20 1.63555 22.7 80.73
7 * 127.969 7.00 79.94
8 42.722 9.03 1.43387 95.1 66.26
9 63.502 0.15 63.89
10 47.420 5.00 1.84666 23.8 60.69
11 30.914 (variable) 50.56
12 108.606 1.80 1.80000 29.8 35.18
13 24.091 4.96 1.80809 22.8 31.93
14 42.619 (variable) 30.99
15 (Aperture) ∞ 0.15 23.04
16 44.998 4.37 1.43387 95.1 23.32
17 -65.820 2.08 1.84666 23.8 23.23
18 -153.409 1.92 23.35
19 -89.454 1.80 1.88300 40.8 26.28
20 33.693 13.00 1.68893 31.1 26.91
21 -33.565 0.50 28.53
22 -40.429 1.80 1.88300 40.8 28.24
23 71.528 1.74 29.50
24 52.398 8.51 1.72151 29.2 28.40
25 -24.165 1.40 1.80809 22.8 28.77
26 89.223 4.05 30.75
27 -48.755 1.80 1.59282 68.6 30.92
28 63.474 8.86 1.67270 32.1 36.35
29 -42.321 0.15 37.60
30 127.241 4.55 1.65412 39.7 40.75
31 -191.324 2.42 41.05
32 ∞ 2.20 1.51633 64.1 41.52
33 ∞ (variable) 41.71
Image plane ∞

Aspheric data 5th surface (diffractive surface)
C1 = -3.69695e-005 C2 = -3.94430e-009 C3 = 3.48364e-012 C4 = -2.04075e-015
C5 = 4.29802e-019

7th page
K = 1.65526e + 000 A4 = -6.03995e-009 A6 = -1.25921e-011 A8 = 2.88737e-015 A10 = -9.96150e-019

Various data focal length 392.12
F number 4.12
Angle of View 3.16
Statue height 21.64
Total lens length 262.14
BF 69.97

(Numerical example 2)
Unit mm
Surface data surface number rd nd vd Effective diameter
1 92.617 12.53 1.48749 70.2 95.25
2 343.518 0.03 1.55184 49.1 94.27
3 (Diffraction) 343.489 0.03 1.49515 18.7 94.26
4 343.461 3.11 1.63555 22.7 94.25
5 * 635.945 0.15 93.57
6 69.921 9.63 1.48749 70.2 86.95
7 124.613 5.42 85.31
8 45.567 9.69 1.43387 95.1 71.95
9 60.992 0.15 68.58
10 49.537 5.00 1.84666 23.8 65.91
11 33.274 (variable) 55.30
12 114.393 1.80 1.80000 29.8 35.11
13 24.277 9.00 1.80809 22.8 31.99
14 40.933 (variable) 29.23
15 (Aperture) ∞ 0.15 23.04
16 41.402 4.72 1.43387 95.1 23.40
17 -55.816 3.24 1.84666 23.8 23.33
18 -140.266 1.49 23.58
19 -76.632 1.80 1.88300 40.8 26.55
20 36.677 10.99 1.68893 31.1 27.40
21 -33.346 0.50 28.75
22 -42.027 1.80 1.88300 40.8 28.49
23 77.260 1.65 29.80
24 53.240 8.03 1.69895 30.1 28.70
25 -26.966 1.40 1.80809 22.8 29.09
26 77.864 3.67 31.12
27 -65.464 1.80 1.59282 68.6 31.31
28 57.369 9.29 1.68893 31.1 36.48
29 -41.570 0.15 37.68
30 79.585 4.10 1.65412 39.7 40.97
31 545.403 2.50 41.05
32 ∞ 2.20 1.51633 64.1 41.31
33 ∞ (variable) 41.49
Image plane ∞

Aspheric data 3rd surface (diffractive surface)
C1 = -2.70457e-005 C2 = -2.93455e-010 C3 = -3.61642e-013 C4 = 2.34170e-016
C5 = -5.55157e-020

5th page
K = 4.48297e + 001 A4 = 2.36168e-008 A6 = -5.72910e-013 A8 = -4.43608e-016 A10 = 7.12124e-020

Various data

Focal length 392.13
F number 4.12
Angle of View 3.16
Statue height 21.64
Total lens length 262.14
BF 74.92

The values of the conditional expressions in each embodiment are shown in Table 1 below.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

LF ... front group, LR ... rear group, Ldam ... diffraction optical element, S ... aperture stop, 1st diffraction grating, 2nd diffraction grating, 102 ... diffraction optical part, 103 .. Refraction optical part, 104 .. Glass substrate, 105 .. Aspherical shape

Claims (10)

The front group has a front group, an aperture stop, and a rear group in order from the object side, and the front group has a diffractive optical unit formed by stacking a plurality of diffraction gratings in order from the object side, and abnormal partial dispersion to reduce chromatic aberration. An optical element having a refractive optical part including a solid material having characteristics, and an optical system having a focusing part,
The optical system, wherein the refractive optical part has an aspherical shape in which an optical surface not joined to the diffractive optical part faces air. The diffractive optical part is disposed at a position that satisfies the following conditional expression (1), and has a refractive power that satisfies the following conditional expression (2):
The optical system according to claim 1, wherein the refractive optical unit is made of the solid material that satisfies the following conditional expressions (3) and (4).
0.5 <| h | <1.0 -------------------- (1)
0.01 <| f / fdoe | <0.1 --------------- (2)
When ΔθgF = θgF − (− 1.665 × 10 ⁻⁷ × νd ³ + 5.213 × 10 ⁻⁵ × νd ² −5.656 × 10 ⁻³ × νd + 0.7278) 0.02 <ΔθgF <0. 25 ----------------- (3)
10 <νd <40 ----------------------- (4)
Here, h is the height from the optical axis when passing through the diffractive surface of the diffractive optical part of the diffractive optical element of the on-axis paraxial light beam incident at a height 1 from the optical axis parallel to the optical axis of the optical system. F is the focal length of the entire optical system, fdoe is the focal length of the diffraction surface of the diffractive optical element of the diffractive optical element, and νd is a solid material represented by νd = (nd-1) / (nF-nC). Abbe number, θgF represents the partial dispersion ratio of the solid material represented by θgF = (ng−nF) / (nF−nC), and ΔθgF represents the abnormal partial dispersion ratio represented by the above equation. The optical system according to claim 1 or 2, wherein the aspherical shape satisfies the following conditional expression (5).
Δdb = ((1 / R) * r ² / (1 + √ (1− (1 + k) * (r / R) ² )) + B * r ⁴ + C * r ⁶ + D * r ⁸ + E * r ¹⁰ +. When ((1 / R) * r ² / (1 + √ (1- (r / R) ² )))

0.03 <| Δdb_max / danm | <0.30 -------- (5)
Here, R is a paraxial radius of curvature, r is a height from the optical axis in a direction perpendicular to the optical axis, k is a conic constant, B, C, D, E. Δdb is the amount of aspherical component defined by the above equation, and Δdb_max is the maximum amount of aspherical component within the effective radius of the light beam passing through the optical surface on the optical surface provided with the aspherical surface defined by Δdb. , Danm represents the thickness of the refractive optical part of the diffractive optical element on the optical axis. The optical system according to any one of claims 1 to 3, wherein the following conditional expression (6) is satisfied when a focal length of the refractive optical unit is set to fanm.
0.2 <| fanm / f | <5.0 --------------- (6) The diffractive optical section is formed by closely bonding two diffraction gratings 1 and 2 made of materials with different dispersions at the grating surfaces of each other, and the wavelengths of the Fraunhofer line g-line, F-line, d-line, and C-line. Λg, λF, λd, λC, refractive indexes of the diffraction gratings 1 and 2 at the wavelengths of g-line, F-line, d-line, and C-line are n1g, n2g, n1F, n2F, n1d, n2d, n1C, n2C, and diffraction grating Is obtained by dividing the difference in optical path length between the convex and concave portions of the diffraction grating by the respective wavelengths with respect to the M-th order diffracted light in the vicinity of the design order, by M (λg ) = {Dh × (n1g−n2g)} / λg, M (λF) = {dh × (n1F−n2F)} / λF, M (λd) = {dh × (n1d−n2d)} / λd, M ( When λC) = {dh × (n1C−n2C)} / λC, the following conditional expressions (7) to (1 ) Optical system according to any one of claims 1 to 4, characterized by satisfying the.
1.5 ≦ n1d ≦ 1.8 -------------------- (7)
40 ≦ νd1 = (n1d * 1) / (n1F * n1C) ≦ 80 ------ (8)
θ1gF = (n1g * n1F) / (n1F * n1C), and Δθ1gF = θ1gF − (− 1.665 × 10 ⁻⁷ × νd1 ³ + 5.213 × 10 ⁻⁵ × νd1 ² −5.656 × 10 ⁻³ × When νd1 + 0.7278)

-0.05 <Δθ1gF <0.05 ---------------- (9)
1.3 ≦ n2d ≦ 1.6 -------------------- (10)
5 ≦ νd2 = (n2d * 1) / (n2F * n2C) ≦ 30 ------- (11)
θ2gF = (n2g * n2F) / (n2F * n2C), and Δθ2gF = θ2gF − (− 1.665 × 10 ⁻⁷ × νd2 ³ + 5.213 × 10 ⁻⁵ × νd2 ² −5.656 × 10 ⁻³ × νd2 + 0.7278)

-0.50 <[Delta] [theta] 2gF <-0.02 -------------- (12)
0.01 <n1d * n2d <0.5 ----------------- (13)
0.01 <dh / p ≦ 0.1 ------------------- (14)
0.92 ≦ {M (λg) + M (λF) + M (λd) + M (λC)} / (4 * M) ≦ 1.08 ----------------- ----------- (15)   In the diffractive optical part, the diffraction grating 1 is made of a resin material or a resin material containing inorganic fine particles of any one of Al, Zr, Y and oxides, composites, and mixtures thereof, and the diffraction grating 2 is made of resin material or ITO or 6. The optical system according to claim 5, wherein the optical system is made of a resin material containing inorganic fine particles of any one of Ti, Nr, Cr and oxides, composites, and mixtures thereof.   The optical element is formed using a mold, and the diffraction grating 1 is formed on a glass substrate, and the diffraction grating 2 is formed so as to cover the formed diffraction grating 1 to form the diffraction optical unit. The optical system according to claim 5, wherein a refractive optical part is formed on an optical surface that is not on the glass substrate side of the diffractive optical part. An optical element obtained by bonding a diffractive optical part formed by laminating a plurality of diffraction gratings and a refractive optical part including a solid material having anomalous partial dispersion characteristics,
The optical element, wherein the refractive optical part has an aspherical shape in which an optical surface not joined to the diffractive optical part faces air. 9. The optical element according to claim 8, wherein the refractive optical part is made of the solid material that satisfies the following conditional expressions (3) and (4).
When ΔθgF = θgF − (− 1.665 × 10 ⁻⁷ × νd ³ + 5.213 × 10 ⁻⁵ × νd ² −5.656 × 10 ⁻³ × νd + 0.7278) 0.02 <ΔθgF <0. 25 ----------------- (3)
10 <νd <40 ----------------------- (4)
Here, νd is the Abbe number of the solid material represented by νd = (nd-1) / (nF-nC), and θgF is the solid material represented by θgF = (ng-nF) / (nF-nC). The partial dispersion ratio, ΔθgF, represents the abnormal partial dispersion ratio represented by the above equation. The optical element according to claim 8, wherein the aspherical shape satisfies the following conditional expression (5).
Δdb = ((1 / R) * r ² / (1 + √ (1− (1 + k) * (r / R) ² )) + B * r ⁴ + C * r ⁶ + D * r ⁸ + E * r ¹⁰ +. (1 / R) * r ² / (1 + √ (1- (r / R) ² )) 0.03 <| Δdb_max / danm | <0.30 −−−−−−−−−− (5)
Here, R is a paraxial radius of curvature, r is a height from the optical axis in a direction perpendicular to the optical axis, k is a conic constant, B, C, D, E. Δdb is the amount of aspherical component defined by the above equation, and Δdb_max is the maximum amount of aspherical component within the effective radius of the light beam passing through the optical surface on the optical surface provided with the aspherical surface defined by Δdb. , Danm represents the thickness of the refractive optical part of the diffractive optical element on the optical axis.